Project Summary/Abstract Sexual minorities continue to face mental health disparities ? recent national data show that 37% of sexual minority adults had a mental illness. Stigma can exist internally among sexual minorities when they internalize negative attitudes about themselves via pervasive societal homophobia. Internalized stigma is a key determinant of mental health disparities for sexual minorities; however, the implicit or subconscious dimension of internalized stigma has been almost completely overlooked in research and clinical practice, perhaps because there is not a valid and reliable instrument to measure this phenomenon. To address gaps in this area, this project has three aims: (1) Creating an implicit internalized sexual minority stigma instrument using an Affect Misattribution Procedure (AMP) approach; (2) Evaluating the reliability (internal consistency and test-retest reliability) and validity (convergent, discriminant, and predictive validity) of the implicit internalized sexual minority stigma version of the AMP; and (3) Examining concurrent and predictive relationships between implicit internalized sexual minority stigma and outcomes of depressive symptoms and anxiety symptoms. Developing the measure will involve a mixed-methods approach: generating visual stimuli for the measure (i.e., images of heterosexual and same-sex couples), evaluating couple images to balance across groups, interviewing experts to identify valence response options, programming the measure for online administration, pilot-testing the preliminary measure with a sample of 100 sexual minority adults, and dropping unreliable response dimensions based on pre-testing. Evaluating the psychometrics of the final measure will involve online data collection from 400 sexual minority adults across two waves and administration of measures of implicit and explicit internalized stigma. Examining relations between implicit internalized stigma and mental health outcomes will use the sample of 400 sexual minority adults assessed at two waves, and data collection on depressive symptoms, anxiety symptoms, and relevant covariates to isolate the effect of implicit internalized stigma on mental health. Analyses, will involve reliability analysis, pairwise correlations, and cross-lagged panel modeling. This project is innovative and significant because it has the potential to shift research and clinical practice, which has almost exclusively focused on explicit internalized stigma. The instrument could be used in epidemiological research to understand implicit internalized stigma as a determinant of mental health disparities. It could also be used in intervention research and clinical practice as a target mechanism to improve mental health among sexual minorities.